The present invention relates to a method and an apparatus for detecting and compensating disk tilt, more particular to a technique for pickups to solve the reading-error problem induced by coma as there are optical disk tilts in the radial and tangential directions.
Optical disks include a transparent substrate having a recording layer where data is recorded and stored. Data can be stored on the recording layer in various forms, including pits, marks, and magneto-optic domains. In an optical disk system, a laser beam is focused by an objective lens through the transparent substrate and onto the stored data. The laser beam is then reflected back through the same objective lens for focusing. Since surface defects on the disk, such as dust particles and scratches, can have dimensions on the order of the focused spot size of the laser beam, the laser beam is typically focused onto the rear surface of the disk substrate to ensure that any surface defects will be out of focus with respect to the recording layer containing the data. Any spherical aberration caused by focusing the laser beam through the substrate will generally be corrected by the design of the objective lens.
Typically, the optical disk is not perfectly flat, and any local deviations from flatness appear as a slight tilt of the front surface of the disk with respect to the incident beam. In addition, when the optical disk placed on a turntable of a player is warped, the front surface of the disk is tilted relative to the optical axis of the focused laser beam, and coma aberration occurs. Additional tilt components can be caused by spindle misalignment or disk droop. The disk tilt causes a degradation of the focused spot quality of the laser beam, which results in a decrease in the carrier-to-noise ratio during readout, an increase in crosstalk and intersymbol interference, and a reduction in recording sensitivity.
Future generations of optical disks will most likely utilize shorter wavelengths and higher numerical aperture objective lenses, both of which increase the area data density of the disks. Unfortunately, the sensitivity to disk tilt increases if the wavelength and numerical aperture quantities are adjusted to meet a higher data density. Several systems have been proposed that attempt to dynamically correct for the effects of disk tilt, for example, by tilting the objective lens or the entire optical head.
As a prior art, U.S. Pat. No. 5,065,380 discloses a tilt-detection method by adapting a pickup with three laser beams in the radial direction of an optical disk. The method measures only the radial tilt but not the tangential tilt, and it can not resist the tracking motion of the pickup.
U.S. Pat. No. 5,523,989 discloses another pickup with a tilt-compensation method which measures the disk tilt according to the push-pull signal (the interference between a zero-order diffracted light beam and +1, xe2x88x921 first-order diffracted light beams reflected by the optical disk). The pickup compensates the coma by using a servo actuator. The pickup of the invention needs an additional lens and its size is larger than that of a traditional pickup.
Disclosed in U.S. Pat. No. 5,805,543, a method adapts two consecutive laser pulses to measure the disk tilt. Its calculation is too laborious to be realized on line. One type of tilt sensor measures tilt with respect to the disc surface, but only after the information beam is tracking and focused in an information track. Such systems are described in U.S. Pat. No. 5,206,848 to Kusano. Because these devices require relative stability between the optical recording actuator and the disc, they are ineffective for measuring inertial tilt during rapid actuator movements.
An apparatus for producing a tilt error signal representative of the tilt of an optical disk is disclosed in U.S. Pat. No. 5,805,543. The apparatus includes a source of laser beam for focusing a laser beam onto the disk and reflecting such beam from the disk, a beam splitter positioned to receive the reflected light beam and to direct the light beam in a first direction, and a structure for separating the reflected light beam from the beam splitter into at least four portions and for producing detection signals for each portion. The apparatus further includes circuitry responsive to the detection signals for producing first and second tracking error signals, the first and second tracking error signals being produced by different combinations of the detection signals and both being sensitive to cross-track diffraction, and circuitry responsive to the first and second tracking error signals for producing the tilt error signal. The apparatus has no actuator to compensate the disk tilt.
A combined sensor for measuring tilt and tracking position of a lens holder with respect to an optical recording actuator base is disclosed in U.S. Pat. No. 5,732,054. A light source and two photosensitive bi-cell detectors are secured to the actuator base so that a beam from the light source strikes the detectors, and an optical slot or flag is secured to the lens holder between the light source and the detectors, for creating an image on the bi-cell detectors. Output from the bi-cell detectors is converted into information on tilt and position of the lens holder relative to the actuator base.
The main objective of the invention is to provide a method and a pickup to detect the radial and tangential tilts of an optical disk and compensate the coma induced by the disk tilt. The pickup adapts a two-dimension (2-D) grating to produce a plurality of laser beams for detection. These laser beams include a zero-order (0-order) diffracted laser beam, +1, xe2x88x921 first-order radial diffracted laser beams and +xe2x88x92 first-order (+1-order and xe2x88x921-order) tangential diffracted laser beams. In addition, two detectors are adapted in the pickup to measure the interference of the reflected laser beams corresponding to the radial and tangential tilts. According to the radial and tangential tilts of the disk, a reflective mean is controlled and actuated to change the incident angle of the reading laser beam of the pickup for the compensation of the coma induced by the disk tilt.
The other objective of the invention is to provide a simple actuator which adjusts the reflection angle of a reflective mean to change the incident angle of the reading laser beam of the pickup for the compensation of the coma induced by the disk tilt. The actuator includes piezoelectric driving devices or electromagnetic driving devices.